There are already known various constructions of interferometric measuring systems, among them, for instance, so-called coherence multiplexed systems such as those constructed to be employed as laser vibration sensors or optical coherence multiplexers. It is usually not possible to use such known techniques such as high frequency laser modulation (greater than 10 MHz) or special coatings on the laser mirrors in coherence multiplexed systems since they typically produce coherence lengths of less than 100 micrometers, which is insufficient for such applications. Therefore, a short coherence length laser diode, with a coherence length in the range of about one to ten centimeters, is often an important component of the interferometric measuring arrangements of the latter type. This limited yet still significant coherence length is typically achieved by constructing the laser diode in such a manner as to simultaneously emit light at several discrete optical frequencies. However, when such a laser diode is used in a laser vibration sensor, in an optical coherence multiplexer or in similar applications, there is encountered the disadvantage that the fringe visibility profile is discontinuous at periodic intervals of the aforementioned 1-10 continuation range as a result of the presence of the aforementioned discrete frequencies in the laser beam. Such discontinuities then result in signal dropout at periodic intervals of path mismatch between the two arms of the interferometer employed in the system of this kind. This, of course, is very disadvantageous because at least some of the information to be gathered, transmitted and/or evaluated is lost.
Accordingly, it is a general object of the present invention to avoid the disadvantages of the prior art.
More particularly, it is an object of the present invention to provide an interferometric measuring arrangement which does not possess the disadvantages of the known arrangements of this kind.
Still another object of the present invention is to develop a smoothing device for use in the arrangement of the type here under consideration, which device enhances the interferometric fringe visibility in the optical output of such an arrangement.
It is yet another object of the present invention to devise a smoothing device of the above type in such a manner as to operate in a purely passive mode after or even without an initial adjustment.
A concomitant object of the present invention is design the device of the above type in such a manner as to be relatively simple in construction, inexpensive to manufacture, easy to use, and yet reliable in operation.